Beams of penetrating ionizing radiation such as neutrons and x-rays or gamma rays are useful for nondestructive inspection of material and for medical diagnosis. In one mode of application, penetrating radiation is used to make a transmission image of an object in order to provide a projection of the object's internal structure onto an image plane. The image is made by irradiating the object with radiation from a substantially point source and positioning an imaging detector to detect and record the radiation transmitted through the object. The imaging detector thereby creates an image of the object's internal structure, as discussed, for example, in H. Berger, Neutron Radiography: Methods, Capabilities, and Applications, New York, Elsevier (1965) pp. 47-80, incorporated herein by reference.
Detection of penetrating radiation generally requires conversion of the energy carried by the penetrating particles, or quanta, into visible light. The light is recorded at the image plane of a detection system to form an image suitable for visual inspection or computer analysis. For example, one known form of detection system for X-rays includes an inorganic crystal made of NaI(Tl), CsI, or LiI optically coupled to an image intensifier. When an X-ray of relatively low energy enters the NaI crystal and is fully absorbed, an average of 43 visible photons/keV of X-ray energy is emitted. Some of the visible photons are captured by the photocathode of the image intensifier to give rise to photoelectrons. Electron multiplication occurs in the image intensifier, and the electrons produced in the image intensifier are directed onto a phosphor screen to produce a visible image.
An analysis of the detection process, applied to X-ray detection, is contained in H. H. Barrett, et al., Radiological Imaging, New York, Academic Press (1981) pp. 582-597, also incorporated herein by reference.
FIG. 1 illustrates a system, such as that just described, in operation. A penetrating particle or quantum, such as a neutron or an X-ray, is shown incident on a thick, optically homogeneous scintillator from the right of the figure. Absorption or scattering of the particle or quantum within the scintillator results in light emission from a small neighborhood, of the order of tens of microns, of the absorption or scattering event. A circular aperture having diameter d is provided with a lens and is placed a distance s.sub.o from a reference plane in the scintillator. The reference plane is parallel to the scintillator face and to the aperture boundary. The lens focuses light passing through the aperture upon a photocathode oriented parallel to the reference plane at a distance s.sub.1 from the lens. The photocathode is placed at the focal point of light rays originating from the intersection of the lens axis and the reference plane. That intersection and the focal point thereby bear an optically conjugate relationship with respect to the lens in the FIG. 1, and the photocathode defines an image plane for the system.
The light generated in the scintillator along a single ray from the source does not, of course, all originate from the reference plane. Light originating from events at points off the reference plane will not be focused at the photocathode, because points off the reference plane are not optically conjugate, with respect to the lens in FIG. 1, to any points in the photocathode plane. Instead the light will be focused in front of or behind the photocathode as respectively indicated by the solid and dotted lines in FIG. 1. As a result the image of a ray in the scintillator is not a point but a spot having an approximate width EQU w.apprxeq.t(d/s.sub.o)[(s.sub.o f)-1].sup.-1 ( 1)
where t is the scintillator thickness, f the lens focal length, and the aperture diameter is small compared with the distance of the aperture from either the image plane or the photocathode. Accordingly, use of a thick scintillator causes loss of resolution by defocussing.
However, the absorption mean free path of radiation generally increases with increasing energy. Accordingly, the scintillator thickness must be increased as the energy of the incident radiation is increased in order to have an acceptable number of particles or X-ray quanta intercepted by the scintillator. Thus, on the one hand, a 2 mm thick single-crystalline plate of 96% enriched .sup.6 LiI(Eu) provides about 95% attenuation of a thermal neutron beam. On the other hand, a 2 cm thickness of typical organic scintillator will only provide about 40% attenuation of a 1 Mev neutron beam. A corresponding increase in absorption mean free path for hard X-rays compared with soft X-rays necessitates the use of thicker scintillators with increasing X-ray energy.
Use of thick scintillator screens, however, gives rise to an additional resolution problem stemming from parallax. The origin of the problem may be understood from FIG. 2 where the aperture and lens of FIG. 1 are replaced by a pinhole.
FIG. 2 shows a thick scintillator crystal having a rear face A'-A. A portion of the front face of the scintillator is viewed through a pinhole P by a photocathode Ph which may belong to an image intensifier. A source S having dimensions small compared with the distance from the source to the scintillator rear face A'-A irradiates the scintillator through its rear face with penetrating radiation. An object, indicated by the arrow X'-X, placed between the source and scintillator face, is radiated by the source S and imaged onto the photocathode, as symbolized by the image Y'-Y. Two rays S1-A and S2-A', having projections intersecting at an apex point V, define one projection of the conical envelope enclosing radiation from the source S that strikes the rear face A'-A of the scintillator. Accordingly, as illustrated in FIG. 2, the source may be used to provide a cone of radiation upon the scintillator rear face.
The ratio of source dimension to source-scintillator distance, and the source-object distance, are selected to provide no more than a pre-selected minimum geometric unsharpness, as explained in IRT Corporation, "Californium-252 Neutron Radiography System, Model CFNR-10: Instruction and Operating Manual," Technical Manual No. OM-6155-8, San Diego, Calif. (1977), incorporated herein by reference, pp. 36-41. The solid angle, centered on the source, intercepted by the scintillator is made large if it is necessary to maximize radiation intensity from the source onto the scintillator.
The parallax effect is illustrated in FIG. 2 by the line from the source intersecting the rear face of the scintillator at B and the front face of the scintillator at B'. As may be seen from the figure, the point B is focused by the pinhole upon the photocathode at a point C, and the point B' is focused by the pinhole upon the photocathode at a different point C'. Radiation from the source S passing through the crystal along the path B-B' will cause scintillations along some length of the path within the scintillator. The scintillations will cause an image to form along a corresponding portion of the path on the photocathode between the respective points C and C'. Radiation incident upon the scintillator at a single point may then be recorded upon the photocathode as a line having appreciable length. This effect is undesirable because in order to have high resolution, with correspondingly sharp images, it is necessary that each quantum of radiation passing through a given point in the object be recorded substantially as a single point.